A stratospheric aerosol climatology from SAGE II and CLAES measurements: 1. Methodology

This paper presents the methods used to produce a global climatology of the stratospheric aerosol using data from two satellite instruments: the Stratospheric Aerosol and Gas Experiment (SAGE II) and the Cryogenic Limb Array Etalon Spectrometer (CLAES). The climatology, which spans from December 1984 to August 1999, includes values and uncertainties of measured extinction and optical depth and of retrieved particle effective radius R_eff, distribution width σ_g, surface area S, and volume V. As a basis for aerosol retrievals, a multiwavelength look-up table (LUT) algorithm was developed that matches the satellite-measured extinction ratios to precomputed ratios that are based on a range of unimodal lognormal size distributions. For cases in which the LUT does not find an acceptable match between measured and precomputed extinction spectra, a different technique called the parameter search technique is utilized. The combination of these two techniques and data from both satellites allows us to retrieve values of R_eff, σ_g, S, and V over a wider range of conditions and from a wider range of wavelengths than used by other methods. This greater wavelength range helps constrain retrieved results, especially in postvolcanic conditions when particle sizes are greatly increased and SAGE II extinction spectra become essentially independent of wavelength. Our method includes an altitude- and time-dependent procedure that uses bimodal size distributions from in situ measurements to estimate bias and uncertainty introduced by assuming a unimodal functional form. Correcting for this bias reduces uncertainty in retrievals of R_eff, S, and V by about 7%, 5%, and 1% (averaged over all altitude bands), leaving remaining uncertainties from the unimodal assumption of about ±18%, ±20%, and ±21%, respectively. Additional uncertainties, resulting from measurement error and spatiotemporal variability, are evaluated by propagating input uncertainties through the retrieval algorithm. In an accompanying paper we report on a climatology of R_eff, S, and V and consider uncertainties in our retrieved values of these parameters. In this paper we examine the sensitivity of our retrievals to refractive index and measurement wavelength. We find, for example, that changing refractive index from a value for the stratospheric temperature of 215 K to that for 300 K can increase retrieved R_eff by ∼7.5%, owing largely to effects at the CLAES 12.82 μm wavelength. When only SAGE II wavelengths are used, corresponding changes in R_eff are much smaller.